Manufacture of LiPO2F2 from POF3 or PF5

ABSTRACT

LiPO 2 F 2 , an electrolyte salt additive for batteries, is manufactured by the reaction of POF 3 , PF 5  or mixtures thereof, with Li 3 PO 4  forming a reaction mixture comprising LiPO 2 F 2 . When POF 3  is applied, the reaction mixture which contains essentially only LiPO 2 F 2  is preferably extracted from the reaction mixture with a solvent which also is applicable as solvent for lithium ion batteries. If PF 5  is applied, then, depending on the molar ratio of PF 5  and Li 3 PO 4 , the reaction mixture also contains LiF and/or LiPF 6 . To isolate pure LiPO 2 F 2  from LiF, the reaction mixture containing essentially only LiPO 2 F 2  and LiF may for example, be extracted with dimethoxyethane, acetone, dimethyl carbonate or propylene carbonate. To isolate pure LiPO 2 F 2  from LiPF 6 , the reaction mixture containing essentially only these constituents is preferably extracted with a solvent which also is applicable as solvent for the LiPF 6  in lithium ion batteries to dissolve and remove LiPF 6 .

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. national stage entry under 35 U.S.C. §371 of International Application No. PCT/EP2011/063106 filed Jul. 29, 2011, which claims benefit of the priorities of European patent application N° 10171881.5 filed Aug. 4, 2010, and of European patent application N° 10188108.4 filed on Oct. 19, 2010, the whole content of each of these applications being incorporated herein for all purposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for the manufacture of LiPO₂F₂; more specifically, it relates to a method for the manufacture of LiPO₂F₂ comprising a step of reacting a compound which has a P—F bond and is selected from the group consisting of phosphorus pentafluoride (PF₅), phosphoryl fluoride (POF₃), and mixtures thereof, and lithium orthophosphate (Li₃PO₄). The present invention is also directed to the use of POF₃ in the manufacture of LiPO₂F₂.

BACKGROUND OF THE INVENTION

Lithium difluorophosphate, LiPO₂F₂, is useful as electrolyte salt or additive for an electrolyte composition comprising an electrolyte salt for lithium ion batteries. Thus, WO 2008/111367 discloses how to manufacture a mixture of LiPF₆ and LiPO₂F₂ from a halide other than a fluoride, LiPF₆ and water. The resulting salt mixture, dissolved in aprotic solvents, is used as an electrolyte solution for lithium ion batteries. EP-A-2 061 115 describes, as state of the art at that time, the manufacture of LiPO₂F₂ from P₂O₃F₄ and Li compounds, and, as invention, the manufacture of LiPO₂F₂ from LiPF₆ and compounds with a Si—O—Si bond, e.g., siloxanes. US 2008-305402 and US 2008/102376 disclose the manufacture of LiPO₂F₂ from LiPF₆ with a carbonate compound; according to US 2008/102376, LiPF₆ decomposes at 50° C. and above under formation of PF₅ ; according to other publications, PF₅ is only formed at and above the melting point of LiPF₆ (˜190° C.). EP-A-2 065 339 discloses how to manufacture a mixture of LiPF₆ and LiPO₂F₂ from a halide other than a fluoride, LiPF₆ and water. The resulting salt mixture, dissolved in aprotic solvents, is used as an electrolyte solution for lithium ion batteries.

However, the above methods are technically difficult and produce a greater amount of a by-product such as LiF which is not industrially interesting and thus would incur additional cost for the waste treatment. Further the starting material, LiPF₆, is expensive and thus its use increases the production cost. Consequently, there has been a need to develop new processes, which are capable of avoiding the drawbacks indicated above.

DESCRIPTION OF THE INVENTION

Object of the present invention is to provide LiPO₂F₂ in a technically feasible and economical manner. This object and other objects are achieved by the invention as outlined in the patent claims.

According to one aspect of the present invention, LiPO₂F₂ is manufactured by the reaction of Li₃PO₄ and a compound having a P-F bond which compound is selected from the group consisting of POF₃, PF₅, and mixtures thereof. The resulting reaction mixture thereby obtained comprises LiPO₂F₂.

According to one embodiment, LiPO₂F₂ is manufactured from PF₅ and Li₃PO₄. Both are cheap starting materials. Depending on the molar ratio of PF₅ and Li₃PO₄, the reaction mixture may comprise LiF and/or LiPF₆ according to the reaction equations

PF₅+Li₃PO₄→2 LiPO₂F₂+LiF  (I)

2 PF₅+Li₃PO₄→2 LiPO₂F₂+LiPF₆  (II)

Especially the reaction according to equation (II) is advantageous because the LiPF₆ produced is a valuable product per se.

According to another embodiment of the present invention, LiPO₂F₂ is manufactured by the reaction of phosphoryl fluoride (POF₃) and lithium orthophosphate (Li₃PO₄).

2POF₃+Li₃PO₄→3 LiPO₂F₂  (III)

Since no by-product is ideally produced from this reaction, the purity of LiPO₂F₂ is very high even without purification when compared to the process of the prior art where the reaction mixture contains at least one byproduct such as LiF.

Phosphoryl fluoride (POF₃) can be obtained commercially, e.g., from ABCR GmbH & Co. KG, or can be prepared by a known process in the art. For example, POF₃ can be prepared by fluorination of phosphoryl chloride with HF and/or other fluorinating agents, for example, ZnF₂. Alternatively, it may be also prepared by the reaction of H₃PO₄/P₂O₅, HF/H₂O and SO₃/H₂SO₄. Sometimes, the POF₃ obtained may contain PF₅ as impurity, or vice versa, PF₅ may comprise POF₃ as impurity. The advantage of the process of the invention is that even such mixtures can be applied without impact on the yield.

PF₅ may be obtained commercially, e.g., from Praxair, or it may be prepared from PCl₅ and HF or, as described in EP-A-0 0816287, for example from PCl₃, Cl₂ and HF.

Li₃PO₄ is commercially available, e.g., from Strem Chemicals, Inc, Newburyport, USA, or from Chemetall GmbH, Germany. It is a solid with a melting point far above 1000° C.

Consequently, the reactions of the invention are gas-solid reactions or, if a solvent for POF₃ or PF₅, respectively, is applied, a gas-liquid-solid reaction or a liquid-solid reaction.

Preferably, the reaction between PF₅ and Li₃PO₄, between POF₃ and Li₃PO₄ and between mixtures of POF₃ and PF₅, respectively, and Li₃PO₄ is performed in the absence of water or moisture. Thus, the reaction may be performed at least for a part of its duration in the presence of an inert gas; dry nitrogen is very suitable, but other dry inert gases may be applied, too. The reaction can be performed in an autoclave-type vessel or in other reactors. It is preferred to perform the reaction in apparatus made from steel or other materials resistant against corrosion, e.g., in reactors made of or clad with Monel metal.

Li₃PO₄ is preferably applied in the form of small particles, e.g., in the form of a powder. If desired, it can be dried before introducing it into the reaction with POF₃, PF₅ and mixtures thereof.

The reactants POF₃ or PF₅, respectively, can be introduced into the reaction in gaseous form or in the form of a solution in suitable aprotic organic solvents. Suitable solvents are, for example, ether compounds, e.g., diethyl ether, and organic solvents which are useful as solvents in lithium ion batteries; many examples of such solvents, for example, especially organic carbonates, but also lactones, formamides, pyrrolidinones, oxazolidinones, nitroalkanes, N,N-substituted urethanes, sulfolane, dialkyl sulfoxides, dialkyl sulfites, acetates, nitriles, acetamides, glycol ethers, dioxolanes, dialkyloxyethanes, trifluoroacetamides, are given below.

In other embodiments, POF₃ is introduced into the reactor in complex form, especially in the form of a donor-acceptor complex such as POF₃-amine complexes. Those complexes include POF₃- pyridine, POF₃-trietylamine, POF₃-tributylamine, POF₃-DMAP(4-(dimethylamino) pyridine), POF₃-DBN(1,5-diazabicyclo[4.3.0]non-5-ene), POF₃-DBU(1,8-diazabicyclo[5.4.0]undec-7-ene), and POF₃-methylimidazole. In specific embodiments, a separate vessel can be used to supply POF₃ to the reactor vessel. PF₅, too, can be introduced in such manner into the reactor.

POF₃, PF₅ and any mixtures thereof are preferably introduced into the reactor in gaseous form or in the form of a solution in an aprotic organic solvent. POF₃, PF₅ and any mixtures thereof are more preferably introduced into the reactor in gaseous form.

Preferably, no HF is added to the reaction mixture. Preferably, no difluorophosphoric acid is added to the reaction mixture. Preferably, equal to or more than 80%, more preferably, equal to or more than 85%, and most preferably, 100% of the P content in LiPO₂F₂ produced originate from PF₅ or POF₃ and Li₃PO₄ introduced into the reaction according to equations (I), (II) and (III), and less than 20% and preferably less than 5% of the P content in LiPO₂F₂ produced originates from added LiPF₆ ; most preferably, no LiPF₆ is added at all.

The reaction time is selected such that the desired degree of conversion is achieved. Often, a reaction time of 1 second to 5 hours gives good results for the reaction between POF₃, PF₅ and any mixtures thereof with Li₃PO₄. For the reaction between POF₃ and Li₃PO₄, a preferred reaction time of 0.5 to 2 hours, most preferably of around 1 hour gives good results. For the reaction between PF₅ or mixtures of PF₅ and POF₃ and Li₃PO₄, a preferred reaction time of 0.5 to 2 hours, most preferably of around 1 hour gives good results, too. The reaction speed is very fast.

The reaction temperature is preferably equal to or higher than 0° C. Preferably, the reaction temperature is equal to or lower than 100° C.

The reaction temperature when reacting PF₅ and Li₃PO₄ is preferably equal to or lower than 70° C., more preferably, it is equal to or lower than 50° C. Still more preferably, it is lower than 50° C., and especially preferably, it is lower than 45° C. Advantageously, the reaction of PF₅ with Li₃PO₄ is performed at a temperature from 15 to 35° C., preferably at a temperature from 20 to 30° C., and most preferably, at ambient temperature.

When POF₃ and Li₃PO₄ are reacted, the reaction temperature is preferably equal to or higher than ambient temperature (25° C.), more preferably, equal to or higher than 50° C. The reaction temperature is preferably equal to or lower than 100° C., more preferably equal to or lower than 90° C. A preferred range of temperature is from 50 to 90° C.

If desired a reactor can be applied with internal heating or cooling means, or external heating or cooling means. It may have, for example, lines or pipes with a heat transfer agent like water.

The reaction between POF₃, PF₅ or their mixtures with Li₃PO₄ may be performed at ambient pressure (1 bar abs.). Preferably, the reaction of POF₃, PF₅ or their mixtures with Li₃PO₄ is performed at a pressure higher than 1 bar (abs.), more preferably at a pressure higher than 3 bar (abs.), most preferably, higher than 5 bar (abs). As the reaction proceeds, POF₃ and PF₅, respectively, are consumed, and the pressure may consequently be decreasing, in an autoclave for example. The upper limit of the pressure during the reaction is not critical. Often, for practical reasons, the pressure is equal to or lower than 30 bar (abs).

The reaction of POF₃ and PF₅ or their mixtures with Li₃PO₄ can be performed batch wise, for example, in an autoclave. The reactor may have internal means, e.g., a stirrer, to provide a mechanical impact on the surface of the solid particles of Li₃PO₄ to remove reaction product from t he surface and provide an unreacted fresh surface. It is also possible to shake or rotate the reactor itself.

Alternatively, the reaction can be performed continuously, for example, in a flow reactor. For example, the Li₃PO₄ may be provided in the form of a bed ; POF₃, PF₅ or their mixtures may be passed through this bed until a “breakthrough” of POF₃ or PF₅ is observed indicating the end of the reaction. If desired, dry inert gas like nitrogen or noble gases may be passed through the Li₃PO₄ bed to remove oxygen, moisture or both before performing the reaction.

If the reaction is performed continuously, for example, Li₃PO₄ may be kept in the form of a bed in a flow reactor, e.g., as a fluidized bed, and POF₃ or PF₅ or their mixtures is continuously passed through the bed. Continuously, POF₃ and/or PF₅ and unreacted Li₃PO₄ may be introduced into the reactor, and continuously, reaction product may be withdrawn from the reactor. Means, for example, moving parts, e.g., stirrers, may be foreseen in the reactor which provide a mechanical impact on the surface of the solid particles to remove reaction product from their surface and to provide unreacted Li₃PO₄.

If desired, the reaction may be performed in a solvent, e.g., an organic polar aprotic solvent. Preferably, the Li₃PO₄ is dispersed therein. While it is not expected that a significant amount of Li₃PO₄ will dissolve in such a solvent, at least the solvent may serve to dissolve reaction products like LiF or LiPF₆ thus making the isolation of LiPO₂F₂ easier. If desired, the POF₃ and PF₅ or their mixtures, respectively, may be introduced into the reaction dissolved in an aprotic polar solvent, for example, in an ether, e.g., in a dialkyl ether, for example, in diethyl ether, or in other solvents, and especially in one of the solvents for lithium ion batteries mentioned below. Since LiPF₆ is much better soluble in these solvents than LiPO₂F₂, the reaction between POF₃ or PF₅ and Li₃PO₄ and the subsequent removal of formed LiPF₆ can be performed in the same reactor in a kind of “1-pot process”. This is especially preferred if a reaction between POF₃ and Li₃PO₄ is performed because ideally, no by-product is formed, and also for the reaction between PF₅ and Li₃PO₄ because the reaction can be performed such (by providing a relatively high molar ratio of PF₅:Li₃PO₄, e.g., from 2 to 4) that the formation of LiF as by-product is suppressed and the formation of LiPF₆ as by-product is increased; this is explained below.

If desired, after termination of the reaction, a vacuum may be applied, or dry inert gas like nitrogen or noble gases may be passed through the LiPO₂F₂ bed, to remove solvents or residual POF₃ or PF₅.

The resulting reaction mixture is present in solid form if no solvent is used. If desired, the solid may be comminuted, e.g., milled, to provide a larger contact surface if it is intended to dissolve constituents of it.

If desired, the LiPO₂F₂ formed can be isolated from the resulting reaction mixture which may comprise LiF and/or LiPF₆.

In the following, the reaction of PF₅ and Li₃PO₄ is explained in more detail. The molar ratio of PF₅ to Li₃PO₄ is preferably equal to or greater than 0.9:1. It is more preferably equal to or greater than 1:1.

Depending on the molar ratio of PF₅ and Li₃PO₄, the reaction with Li₃PO₄ can be influenced in view of the formation of LiF or LiPF₆ as preferred side product.

According to one embodiment, the molar ratio of PF₅ to Li₃PO₄ is equal to or lower than 2:1, more preferably, lower than 2:1. If the molar ratio of PF₅ and Li₃PO₄ is between 0.9:1, preferably 1:1, and 2:1, it is expected that LiF and LiPF₆ are formed and are present in the reaction mixture. The closer the ratio of PF₅ and Li₃PO₄ is to 2:1, the more LiPF₆ is expected to be formed. The presence of LiPF₆ as reaction product has the advantage that it can be separated from LiPO₂F₂ very easily as is shown below because its solubility is much higher than that of LiPO₂F₂ in a lot of organic solvents. The disadvantage is that LiPF₆ is much more sensitive to moisture than LiF. To separate LiPO₂F₂ from LiF, it is preferred to apply a solvent for LiPO₂F₂ as explained below. It is possible and in some embodiments it is preferred that the resulting reaction mixture is heated to decompose LiPF₆ to form LiF and PF₅. For example, if both LiF and LiPF₆ are present as impurities, LiF is provided as single impurity thus making work-up easier.

According to another embodiment, the molar ratio of PF₅ to Li₃PO₄ is equal to or greater than 2:1. It is preferably equal to or lower than 4, more preferably, it is lower than 4, still more preferably, it is equal to or lower than 3. In this embodiment, LiPO₂F₂ is formed containing LiPF₆ as by-product. As mentioned above, LiPF₆ can be removed easily from LiPO₂F₂ by extraction with a solvent.

The molar ratio of POF₃ to Li₃PO₄ is generally equal to or greater than 1.8:1. It is more preferably equal to or greater than 2:1. It is preferably equal to or lower than 5:1. According to one embodiment, the molar ratio of POF₃ to Li₃PO₄ is equal to or lower than 4:1. Preferably, the molar ratio of POF₃ to Li₃PO₄ is equal to or greater than 2 and equal to or lower than 4.

If desired, mixtures comprising Li₃PO₂F₂ and LiPF₆ in any desired ratio can be produced. In this case, Li₃PO₄ and LiF and sufficient PF₅ are introduced into the reaction. PF₅ forms LiPF₆ with the introduced LiF, and it forms LiPO₂F₂ (and some LiPF₆) with the Li₃PO₄ introduced into the reaction.

To summarize,

a) the reaction of Li₃PO₄ with PF₅ according to reaction scheme (I) provides a reaction product which essentially consists of LiPO₂F₂ and LiF

b) the reaction of Li₃PO₄ with PF₅ according to reaction scheme (II) provides a reaction product which essentially consists of LiPO₂F₂ and LiPF₆

c) the reaction of Li₃PO₄ with POF₃ according to reaction scheme (III) provides a reaction product which essentially consists of LiPO₂F₂ with at most minor amounts of impurities, e.g., LiF.

d) the reaction of a starting material comprising Li₃PO₄ and LiF with PF₅ provides mixtures of LiPO₂F₂ and LiPF₆ in any desired ratio.

Main by-products according to a) and b) are LiF and LiPF₆, respectively ; only one of LiF and LiPF₆, or both, may be present. As outlined above, it is possible by properly selecting reaction conditions, especially the molar ratio of starting compounds (molar ratio of POF₃ and PF₅, respectively, to Li₃PO₄) to influence the presence of LiF and LiPF₆ as by-products. In some embodiments, the presence of LiF or LiPF₆ may be desired. In such an embodiment, no further purification may be necessary. In other embodiments, it may be desired to obtain purified LiPO₂F₂, which is free of LiF or LiPF₆.

If desired, the reaction mixture can be treated to obtain purified LiPO₂F₂; to obtain purified LiPO₂F₂, two embodiments are preferred.

According to one embodiment, LiPO₂F₂ can be purified including a step of extracting the reaction product with a solvent. LiPO₂F₂ can be isolated by using a solvent or a solvent mixture which preferably dissolves LiPO₂F₂. This is the preferred way to separate LiPO₂F₂ from mixtures which contain LiPO₂F₂ and LiF as impurity, e.g., when obtained in a reaction mentioned above under a) and c). The dissolved LiPO₂F₂ can be recovered from the solvent by removing it, e.g., by evaporation of the solvent. Optionally, a solution of LiPO₂F₂ in a suitable solvent may directly be applied for the manufacture of an electrolyte for Li ion batteries.

According to another embodiment, LiPO₂F₂ is purified from impurities by applying a solvent or solvent mixture which preferentially dissolves the impurity. This is the preferred way to separate LiPO₂F₂ and LiPF₆, e.g., when a reaction mixture comprising both is obtained in a reaction mentioned above under b). In a preferred embodiment, formed LiPF₆ is extracted with a solvent applicable in lithium ion batteries. In the following, certain solvents will be described which are preferably applied to separate LiPO₂F₂ and LiF by preferentially dissolving LiPO₂F₂.

If the reaction mixture comprises essentially only LiPO₂F₂ and LiF, the separation is best achieved by contacting the reaction mixture with solvents which preferentially dissolve LiPO₂F₂. Aprotic and protic organic and inorganic solvents are suitable, especially polar solvents. The preferred inorganic solvent is water. Organic protic or aprotic solvents can be used for the extraction, too.

Suitable protic organic solvents are alcohols. Alcohols with one, two or three hydroxy groups in the molecule are preferred. Methanol, ethanol, n-propanol, i-propanol, glycol and glycerin are preferred alcohols. Glycol alkyl ethers, e.g., diglycol methyl ether, are also suitable. Also acetone, in its tautomeric form, can be considered as protic solvent.

Aprotic polar solvents are also very suitable for the extraction of LiPO₂F₂ from the reaction mixture. Preferably, the aprotic organic solvent is selected from the group of dialkyl carbonates (which are linear) and alkylene carbonates (which are cyclic), and wherein the term “alkyl” denotes preferably C1 to C4 alkyl, the term “alkylene” denotes preferably C2 to C7 alkylene groups, including a vinylidene group, wherein the alkylene group preferably comprises a bridge of 2 carbon atoms between the oxygen atoms of the —O—C(O)—O— group; ketones, nitriles and formamides. Dimethyl formamide, carboxylic acid amides, for example, N,N-dimethyl acetamide and N,N-diethyl acetamide, acetone, acetonitrile, linear dialkyl carbonates, e.g., dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, cyclic alkylene carbonates, e.g., ethylene carbonate, propylene carbonate, and vinylidene carbonate, are suitable solvents.

It is also possible to use mixtures containing water and one or more organic protic or aprotic solvents. It is preferred that the pH of the water used for extraction, and of water-containing organic solvents applied for extraction, of the LiPO₂F₂ formed in the reaction is selected such that undesired hydrolysis of LiPO₂F₂ is prevented. Especially, the pH is equal to or lower than 7 to prevent hydrolysis. It is preferred to keep the pH at a value of equal to or lower than 7 during the contact of LiPO₂F₂ with the water or the mixture of water and organic solvent or solvents.

Mixtures of water and protic solvents can be applied for the isolation of LiPO₂F₂, for example, mixtures of water and alcohols with 1, 2 or 3 hydroxy groups, e.g., mixtures of water and methanol, ethanol, isopropanol, n-propanol, glycol, glycerin, or diglycol.

Mixtures of water and aprotic organic solvents, especially, polar aprotic solvents, can also be applied, for example, mixtures of water with one of the solvents mentioned above, e.g., with ethylene carbonate or propylene carbonate.

Of course, it also possible to apply mixtures which comprise water, one or more protic organic solvents, and one or more aprotic organic solvents. For example, mixtures containing water, an alcohol like methanol, ethanol or i-propanol, and a nitrile, for example, acetonitrile, or propylene carbonate, can be applied. The content of water in these mixtures is preferably between 1 and 99% by weight. Aqueous or protic solvents may for example be applied when LiPO₂F₂ is prepared from PF₅ and Li₃PO₄.

Dimethyl carbonate and propylene carbonate are among the preferred solvents for reaction mixtures which essentially consist of LiPO₂F₂ and LiF because LiPO₂F₂ is at least fairly soluble in these solvents while LiF is essentially insoluble. Other very suitable solvents to extract LiPO₂F₂ from reaction mixtures comprising LiF obtained by the reaction of POF₃, PF₅ or their mixtures with Li₃PO₄ as main impurity are ethylene carbonate (EC), ethyl methyl carbonate (EMC), ethyl acetate, diethyl carbonate, a mixture of dimethyl carbonate and propylene carbonate (PC), acetonitrile, dimethoxyethane and acetone. The solubility of LiPO₂F₂ in these solvents at ambient temperature is compiled in the following table 1.

TABLE 1 Solubility of LiPO₂F₂ in certain solvents Solubility of LiPO₂F₂ Solvent [g/100 g solvent] Diethyl carbonate 0.4 Dimethyl carbonate/propylene 0.4 carbonate (1:1 v/v) Acetonitrile 2.8 Dimethoxyethane 37 Acetone 20

The solubility of LiPO₂F₂ in acetonitrile and especially in dimethoxyethane and acetone is remarkably high. Acetone is not very well suited as a solvent for Li ion batteries, but it may advantageously be used for the purification of LiPO₂F₂ because it has a very high solubility for LiPO₂F₂ and a very low solubility for LiF. Thus, mixtures comprising LiF and LiPO₂F₂ can easily be separated by dissolving the LiPO₂F₂ in acetone and filtration to remove solid LiFLiPO₂F₂ can be recovered from its solutions in acetone, for example, by evaporation of the acetone.

The solubility of LiPO₂F₂ in dimethoxyethane is even higher than in acetone. Dimethoxyethane was considered as solvent or solvent additive for Li ion batteries. Thus, dimethoxyethane—which also dissolves LiF at most in negligible amounts—can be used for the purification of LiPO₂F₂ as described above in view of the use of acetone, and it can even be applied to raise the solubility of LiPO₂F₂ in Li ion battery solvents.

Solutions of LiPO₂F₂ in dimethyl carbonate, propylene carbonate and mixtures—which dissolve LiF at most in negligible amounts—are especially suitable for the manufacture of battery electrolytes.

For the isolation of LiPO₂F₂ from LiF or LiPF₆, and especially, if the reaction mixture contains LiPO₂F₂ and LiPF₆ as main impurity, water-free solvents are preferably applied.

This preferred embodiment—the use of essentially waterfree solvents for working up the reaction mixture obtained in the reaction between Li₃PO₄ and POF₃, PF₅ or their mixtures—will be described now in detail.

If the reaction mixture contains essentially only LiPO₂F₂ and LiF, it is preferred to apply solvents which preferentially dissolve LiPO₂F₂.

If the reaction mixture contains essentially only LiPO₂F₂ and LiPF₆, it is preferred to apply solvents which preferentially dissolve LiPF₆.

It was surprisingly found that certain solvents can be applied successfully for both purposes; namely to dissolve LiPO₂F₂ when LiF is present as main impurity, and to dissolve preferentially LiPF₆ if it is the main impurity contained in the reaction mixture comprising LiPO₂F₂ as main product. It was found that LiF is only very sparingly soluble in aprotic organic solvents and that LiPF₆ has a comparably good solubility while the solubility of LiPO₂F₂ is in between.

Solvents for both purposes which are generally aprotic polar organic solvents, are known. Solvents which are useful as electrolyte solvents in lithium ion batteries can be applied. They are preferred because they would not have a detrimental effect on battery electrolytes or could even be used to provide battery electrolytes. Such solvents are generally known. Preferably, a solvent suitable as electrolyte solvent in lithium ion batteries is applied to extract LiPF₆.

In the following, preferred organic aprotic solvents for the workup of reaction mixtures are presented in detail.

Organic carbonates, especially dialkyl carbonates, e.g., dimethyl carbonate or diethyl carbonate, methyl ethyl carbonate, alkylene carbonate, e.g., ethylene carbonate or propylene carbonate, fluorinated solvents, e.g., mono-, di-, tri- and/or tetrafluoroethylene carbonate, are very suitable. Instead or additionally, the extraction of LiPO₂F₂ from mixtures with LiF or, respectively, of LiPF₆ from mixtures comprising LiPO₂F₂ may be performed with other solvents, for example, lactones, formamides, pyrrolidinones, oxazolidinones, nitroalkanes, N,N-substituted urethanes, sulfolane, dialkyl sulfoxides, dialkyl sulfites, as described in the publication of M. Ue et al. in J. Electrochem. Soc. Vol. 141 (1994), pages 2989 to 2996, or trialkylphosphates or alkoxyesters, as described in DE-A 10016816.

Alkyl carbonates with linear and branched alkyl groups and alkylene carbonates are especially suitable for preferentially dissolving LiPO₂F₂ in mixtures comprising LiF, and of LiPF₆ in mixtures comprising LiPO₂F₂, respectively, for example, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate (EMC), diethyl carbonate, and propylene carbonate (PC), see EP-A-0 643 433. Pyrocarbonates are also useful, see U.S. Pat. No. 5,427,874. Alkyl acetates, for example, ethyl acetate, N,N-disubstituted acetamides, sulfoxides, nitriles, glycol ethers and ethers are useful, too, see EP-A-0 662 729. Often, mixtures of these solvents are applied. Dioxolane is a useful solvent, see EP-A-0 385 724. For lithium bis-(trifluoromethansulfonyl)imide, 1,2-bis-(trifluoracetoxy)ethane and N,N-dimethyl trifluoroacetamide, see ITE Battery Letters Vol. 1 (1999), pages 105 to 109, are applicable as solvent. In the foregoing, the term “alkyl” preferably denotes saturated linear or branched C1 to C4 alkyl groups; the term “alkylene” denotes preferably C2 to C7 alkylene groups, including a vinylidene group, wherein the alkylene group preferably comprises a bridge of 2 carbon atoms between the oxygen atoms of the —O—C(O)—O— group, thus forming a 5-membered ring.

Fluorosubstituted compounds, for example, fluorinated carbonic esters which are selected from the group of fluorosubstituted ethylene carbonates, fluorosubstituted dimethyl carbonates, fluorosubstituted ethyl methyl carbonates, and fluorosubstituted diethyl carbonates are also suitable solvents for dissolving LiPO₂F₂ or LiPF₆, respectively. They are applicable in the form of mixtures with non-fluorinated solvents. The non-fluorinated organic carbonates mentioned above are for example very suitable.

Preferred fluorosubstituted carbonates are monofluoroethylene carbonate, 4,4-difluoro ethylene carbonate, 4,5-difluoro ethylene carbonate, 4-fluoro-4-methyl ethylene carbonate, 4,5-difluoro-4-methyl ethylene carbonate, 4-fluoro-5-methyl ethylene carbonate, 4,4-difluoro-5-methyl ethylene carbonate, 4-(fluoromethyl)-ethylene carbonate, 4-(difluoromethyl)-ethylene carbonate, 4-(trifluoromethyl)-ethylene carbonate, 4-(fluoromethyl)-4-fluoro ethylene carbonate, 4-(fluoromethyl)-5-fluoro ethylene carbonate, 4-fluoro-4,5-dimethyl ethylene carbonate, 4,5-difluoro-4,5-dimethyl ethylene carbonate, and 4,4-difluoro-5,5-dimethyl ethylene carbonate; dimethyl carbonate derivatives including fluoromethyl methyl carbonate, difluoromethyl methyl carbonate, trifluoromethyl methyl carbonate, bis(fluoromethyl) carbonate, bis(difluoro)methyl carbonate, and bis(trifluoro)methyl carbonate ; ethyl methyl carbonate derivatives including 2-fluoroethyl methyl carbonate, ethyl fluoromethyl carbonate, 2,2-difluoroethyl methyl carbonate, 2-fluoroethyl fluoromethyl carbonate, ethyl difluoromethyl carbonate, 2,2,2-trifluoroethyl methyl carbonate, 2,2-difluoroethyl fluoromethyl carbonate, 2-fluoroethyl difluoromethyl carbonate, and ethyl trifluoromethyl carbonate; and diethyl carbonate derivatives including ethyl (2-fluoroethyl) carbonate, ethyl (2,2-difluoroethyl) carbonate, bis(2-fluoroethyl) carbonate, ethyl (2,2,2-trifluoroethyl) carbonate, 2,2-difluoroethyl 2′-fluoroethyl carbonate, bis(2,2-difluoroethyl) carbonate, 2,2,2-trifluoroethyl 2′-fluoroethyl carbonate, 2,2,2-trifluoroethyl 2′,2′-difluoroethyl carbonate, and bis(2,2,2-trifluoroethyl) carbonate.

Carbonic esters having both an unsaturated bond and a fluorine atom (hereinafter abbreviated to as “fluorinated unsaturated carbonic ester”) can also be used as solvent to remove LiPF₆ from its mixture with LiPO₂F₂ or to dissolve LiPO₂F₂ to separate it from impurities, e.g., impurities like LiF. The fluorinated unsaturated carbonic esters include any fluorinated unsaturated carbonic esters that do not significantly impair the advantages of the present invention.

Examples of the fluorinated unsaturated carbonic esters include fluorosubstituted vinylene carbonate derivatives, fluorosubstituted ethylene carbonate derivatives substituted by a substituent having an aromatic ring or a carbon-carbon unsaturated bond, and fluorosubstituted allyl carbonates.

Examples of the vinylene carbonate derivatives include fluorovinylene carbonate, 4-fluoro-5-methylvinylene carbonate and 4-fluoro-5-phenylvinylene carbonate.

Examples of the ethylene carbonate derivatives substituted by a substituent having an aromatic ring or a carbon-carbon unsaturated bond include 4-fluoro-4-vinylethylene carbonate, 4-fluoro-5-vinylethylene carbonate, 4,4-difluoro-4-vinylethylene carbonate, 4,5-difluoro-4-vinylethylene carbonate, 4-fluoro-4,5-divinylethylene carbonate, 4,5-difluoro-4,5-divinylethylene carbonate, 4-fluoro-4-phenylethylene carbonate, 4-fluoro-5-phenylethylene carbonate, 4,4-difluoro-5-phenylethylene carbonate, 4,5-difluoro-4-phenylethylene carbonate and 4,5-difluoro-4,5-diphenylethylene carbonate.

Examples of the fluorosubstituted phenyl carbonates include fluoromethyl phenyl carbonate, 2-fluoroethyl phenyl carbonate, 2,2-difluoroethyl phenyl carbonate and 2,2,2-trifluoroethyl phenyl carbonate.

Examples of the fluorosubstituted vinyl carbonates include fluoromethyl vinyl carbonate, 2-fluoroethyl vinyl carbonate, 2,2-difluoroethyl vinyl carbonate and 2,2,2-trifluoroethyl vinyl carbonate.

Examples of the fluorosubstituted allyl carbonates include fluoromethyl allyl carbonate, 2-fluoroethyl allyl carbonate, 2,2-difluoroethyl allyl carbonate and 2,2,2-trifluoroethyl allyl carbonate.

The extraction of LiPO₂F₂ from mixtures containing LiF as impurity and the extraction of LiPF₆ as impurity from mixtures also containing LiPO₂F₂, respectively, may be performed in a known manner, for example, by stirring the reaction mixture with the solvent (extractant) directly in the reactor, or after removing the reaction mixture from the reactor and optionally crushing or milling, in a suitable vessel, e.g., a Soxhlet vessel. The extraction liquid contains a Li salt and may be further processed.

If the separation process served to remove LiPF₆ from LiPO₂F₂, the liquid phase containing LiPF₆ dissolved in the solvent can be separated from the non-dissolved LiPO₂F₂ in a known manner. For example, the solution can be passed through a filter, or it can be decanted, or the separation can be effected by centrifugation. If desired, LiPF₆ can be recovered by removing the solvent, e.g., by evaporation.

The remaining undissolved LiPO₂F₂ can be stored or can be subjected to further purification treatments to obtain pure solid LiPO₂F₂. This can be performed in a known manner. For example, adhering solvent can be removed by evaporation which may preferably be performed in a vacuum depending on the boiling point of the adhering solvent or solvents.

If the separation process served to dissolve LiPO₂F₂, the solid phase remaining after extraction can be separated in a known manner from the extracting solvent containing dissolved LiPO₂F₂. For example, the solution containing LiPO₂F₂ can be passed through a filter, or it can be decanted, or the separation can be effected by centrifugation. The undissolved residue contains essentially all LiF which can be recovered in pure form for example by re-crystallization.

The dissolved LiPO₂F₂ can be recovered from the solution by evaporation of the solvent to obtain pure solid LiPO₂F₂. This can be performed in a known manner. For example, adhering solvent can be removed by evaporation which may preferably be performed in a vacuum depending on the boiling point of the adhering solvent or solvents.

If the reaction mixture contains LiPO₂F₂ and significant amounts of both LiF and LiPF₆, it is preferred first to remove LiPF₆ with a solvent preferentially dissolving LiPF₆, and then to apply a solvent which preferentially dissolves LiPO₂F₂ over LiF. It is possible to apply the same solvent and to perform a step-wise purification. In the first step, LiPF₆ is dissolved by treating the reaction mixture with the solvent which preferably is one of those mentioned above. Due to the good solubility of LiPF₆ it will be dissolved first and can thus be removed from the reaction mixture. The reaction mixture recovered from the first treatment step is then again treated with a solvent which is preferably one of those mentioned above. Now, LiPO₂F₂ is preferentially dissolved. LiF remains as solid. Dissolved LiPO₂F₂ can then recovered from the solution as mentioned above. The isolated LiPO₂F₂ can be used as additive for the manufacture of lithium ion batteries. It can also be used as additive for Li-sulfur batteries and for Li-oxygen batteries or Li-air batteries.

Isolated solid LiPO₂F₂ can be re-dissolved in any suitable solvent or solvent mixture. The solvents mentioned above, including acetone and dimethoxyethane, are very suitable. Since its main use is as electrolyte salt or salt additive in the field of lithium ion batteries, it may be preferably dissolved in a water-free solvent used for the manufacture of the electrolyte solutions of lithium ion batteries. Such solvents are disclosed above.

A solution of LiPO₂F₂ in propylene carbonate for example contains, under standard conditions (25° C., 1 Bara), up to about 3% by weight of LiPO₂F₂ relative to the total weight of the solution. In other solvents or solvent mixtures, the amount of LiPO₂F₂ which dissolves at a given temperature will vary but can easily be determined by simple tests.

Other highly suitable solvents with a high dissolving power for LiPO₂F₂ (e.g., dimethoxyethane and acetonitrile) are given above.

The advantage of the processes of the invention is that the reaction speed is very high even at ambient temperature. Pure crystalline LiPO₂F₂ can be obtained from cheap starting material, for example, when extracted from the reaction mixture containing LiPO₂F₂ and LiF with dimethyl carbonate or propylene carbonate as solvent and subsequent removal of the solvent, e.g., in a vacuum.

An advantage of using POF₃ is that it can be prepared essentially free of HCl even in chlorine-fluorine exchange reactions. Since the boiling point (b.p.) of POF₃, −40° C., is higher than that of HCl (the b.p. of HCl is −85.1° C.) in contrast to PF₅ (the boiling point of which is −84.4° C. which is similar to that of HCl), a simple distillation or condensation technique under pressure can be used for purification of the POF₃ intermediate product, which makes the present process more economical.

Another aspect of the present invention is the use of POF₃ for the manufacture of LiPO₂F₂.

Should the disclosure of any of the patents, patent applications, and publications that are incorporated herein by reference be in conflict with the present description to the extent that it might render a term unclear, the present description shall take precedence.

EXAMPLES

The following examples will describe the invention in further detail without the intention to limit it.

Example 1 Synthesis and isolation of LiPO₂F₂ using PF₅ as P—F Bond Containing Compound

5 g Li₃PO₄ were given into an autoclave. Gaseous PF₅ was introduced into the reactor. An immediate pressure drop was observed which was allocated to the consumption of PF₅ in the reaction with Li₃PO₄ forming LiPO₂F₂ and LiF. Further PF₅ was introduced into the autoclave until the pressure remained at about 3 to 4 bar (abs.) indicating that no further reaction occurred. After removal of the gaseous phase, the remaining solid had a weight of 12 g and was analyzed with XRD. The characteristic signals of LiPO₂F₂, of LiF and of LiPF₆ were identified. For the isolation of LiPO₂F₂, the solid can be heated to about 200° C. to decompose any LiPF₆ to form LiF and PF₅ ; after removal of the PF₅, the remaining solid essentially consists of LiPO₂F₂ and LiF. The solid is given into a Soxhlet vessel and be extracted with dimethyl carbonate. From the combined solutions, the solvent is removed by evaporation in a rotary evaporator, and the resulting solid is subjected to analysis by XRD, F-NMR and P-NMR.

Analytical data of LiPO₂F₂:

XRD:2-Theta values: 21.5 (strong); 22.0; 23.5; 27.0 (strong); 34.2; 43.2

¹⁹F-NMR (470.94 MHz; solution in D-acetone): −84.25 ppm (doublet, the 2 lines at −83.3 ppm and −85.2 ppm, coupling constant 926 Hz)

³¹P-NMR (202.61 MHz; solution in D-acetone): −19.6 ppm (triplet, the 3 lines at −12.3 ppm, −16.9 ppm and −21.5 ppm; coupling constant 926 Hz).

Example 2 Electrolyte Solution for Lithium Ion Batteries, Lithium-Sulfur Batteries and Lithium-Oxygen Batteries

23 g of LiPO₂F₂, 117 g of LiPF₆, 50 g monofluoroethylene carbonate (“F1EC”) and propylene carbonate (“PP”) are mixed in amount such that a total volume of 1 liter is obtained. The resulting solution contains 0.77 mol of LiPF₆ and 0.23 mol LiPO₂F₂. Thus, the amount of lithium compounds is about 1 mol per liter and corresponds to the concentration of lithium salts commonly used for the batteries, especially lithium ion batteries.

Example 3 Synthesis and Isolation of LiPO₂F₂ using POF₃ as P—F Bond Containing Compound

3.5 g Li₃PO₄ were given into an autoclave. Gaseous POF₃ was introduced into the reactor. An immediate pressure drop was observed which was allocated to the consumption of POF₃ in the reaction with Li₃PO₄ forming LiPO₂F₂. Further POF₃ was introduced from a separate metal container with a final pressure of around 6 atm which decreased continuously. The temperature of the vessel was raised up to 80° C. for one hour, and then the gas connection was interrupted and the reaction mixture was cooled down to room temperature while the excess gas phase was pumped off.

After removal of the gaseous phase, the remaining white solid had a weight of 5.7 g and was analyzed with XRD. The characteristic signals of only LiPO₂F₂ were identified. The solid is given into a Soxhlet vessel and be extracted with dimethyl carbonate. From the combined solutions, the solvent is removed by evaporation in a rotary evaporator, and the resulting solid is subjected to analysis by XRD, F-NMR and P-NMR.

The analytical data of LiPO₂F₂ obtained in example 3 corresponded to those of LiPO₂F₂ obtained in example 1.

Example 4 Electrolyte Solution for Lithium Ion Batteries, Lithium-Sulfur Batteries and Lithium-Oxygen Batteries

117 g of LiPF₆, 23 g of LiPO₂F₂ obtained analogously to example 3, 50 g monofluoroethylene carbonate (“F1EC”) and propylene carbonate (“PP”) are mixed in amount such that a total volume of 1 liter is obtained. The resulting solution contains 0.77 mol of LiPF₆ and 0.23 mol LiPO₂F₂. Consequently, the amount of lithium compounds is about 1 mol per liter and thus corresponds to the concentration of lithium salts commonly used for the batteries, especially lithium ion batteries. 

1. A method for the manufacture of LiPO₂F₂ comprising a step of reacting Li₃PO₄ and a compound having a P—F bond, said compound being selected from the group consisting of POF₃, PF₅, and mixtures thereof, thereby forming a reaction mixture comprising LiPO₂F₂.
 2. The method of claim 1 wherein PF₅ is reacted with Li₃PO₄ thereby forming said reaction mixture comprising LiPO₂F₂.
 3. The method of claim 2 wherein the molar ratio of PF₅ to Li₃PO₄ is equal to or greater than 1 and lower than
 2. 4. The method of claim 2 wherein the molar ratio of PF₅ to Li₃PO₄ is equal to or greater than 2 and lower than
 4. 5. The method of claim 2 wherein the reaction between PF₅ and Li₃PO₄ is performed at ambient temperature.
 6. The method of claim 1 wherein POF₃ is reacted with Li₃PO₄ thereby forming said reaction mixture comprising LiPO₂F₂.
 7. The method of claim 6 wherein the molar ratio of POF₃ to Li₃PO₄ is equal to or greater than 2 and equal to or lower than
 4. 8. The method of claim 6 wherein the reaction between POF₃ and Li₃PO₄ is performed at a temperature from 50 to 90° C.
 9. The method of claim 1 wherein LiF is formed during said reacting step and is present in said reaction mixture comprising LiPO₂F₂. and wherein the LiPO₂F₂ formed is extracted from said reaction mixture comprising LiPO₂F₂ and LiF with a solvent selected from the group consisting of dimethyl carbonate, propylene carbonate, acetonitrile, dimethoxyethane, acetone, and mixtures of two or more therof.
 10. The method of claim 9 wherein LiPF₆ formed during said reacting step is decomposed to form LiF and PF_(S) before extraction of LiPO₂F₂.
 11. The method of claim 1, wherein LiPF₆ is formed during said reacting step and is present in said reaction mixture comprising LiPO₂F₂, and wherein said formed LiPF₆ is extracted with a solvent from the reaction mixture comprising LiPO₂F₂ and LiPF₆.
 12. The method of claim 11 wherein said solvent used to extract LiPF₆ is a solvent suitable as electrolyte solvent in lithium ion batteries.
 13. The method of claim 12 wherein said solvent used to extract LiPF₆ is selected from the group consisting of dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, acetonitrile, dimethoxyethane, acetone, and mixtures of two or more therof.
 14. The method of claim 9 wherein the solvent used to extract LiPO₂F₂ is removed from LiPO₂F₂ to isolate a purified LiPO₂F₂.
 15. A method for the manufacture of LiPO₂F₂ comprising utilizing phosphoryl fluoride (POF₃) as a reactant in a reacting step which forms LiPO₂F₂.
 16. The method of claim 15 wherein POF₃ is introduced into the reacting step in gaseous form.
 17. The method of claim 15 wherein POF₃ is reacted with lithium orthophosphate (Li₃PO₄).
 18. The method of claim 17 wherein the reaction between POF₃ and Li₃PO₄ is performed at a temperature from 50 to 90° C. 